linux-kernel - Re: [00/17] Large Blocksize Support V3

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Date:	Thu, 26 Apr 2007 19:04:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	clameter@....com
Cc:	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

On Tue, 24 Apr 2007 15:21:05 -0700 clameter@....com wrote:

> This patchset modifies the Linux kernel so that larger block sizes than
> page size can be supported. Larger block sizes are handled by using
> compound pages of an arbitrary order for the page cache instead of
> single pages with order 0.

Something I was looking for but couldn't find: suppose an application takes
a pagefault against the third 4k page of an order-2 pagecache "page".  We
need to instantiate a pte against find_get_page(offset/4)+3.  But these
patches don't touch mm/memory.c at all and filemap_nopage() appears to
return the zeroeth 4k page all the time in that case.

So.. what am I missing, and how does that part work?

Also, afaict your important requirements would be met by retaining
PAGE_CACHE_SIZE=4k and simply ensuring that pagecache is populated by
physically contiguous pages - so instead of allocating and adding one 4k
page, we allocate an order-2 page and sprinkle all four page*'s into the
radix tree in one hit.  That should be fairly straightforward to do, and
could be made indistinguishably fast from doing a single 16k page for some
common pagecache operations (gang-insert, gang-lookup).

The BIO and block layers will do-the-right-thing with that pagecache and
you end up with four times more data in the SG lists, worst-case.
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